1. Field of the Invention
This invention relates to a minute structure and a write-once (WORM, or write-once read-many) information recording medium.
2. Description of the Related Art
Recently, the research and development of minute structures with a size ranging from nanometer scale to micrometer scale is carried out in many fields including nanophotonics, high-density recording media, optical elements, and biochemical chips. What is indispensable to those devices utilizing the opto-electronics is a material which is optically transparent in a visible light region and has little optical loss. For this reason, the research and development of a technology for producing a minute structure from a transparent material is carried out briskly.
Zinc oxide is optically transparent in a visible light region and has a property to absorb ultraviolet light. Zinc oxide is used for the purposes, such as LED, transparent transistor, UV cut material, electrophotography, etc.
Examples of the method of forming a zinc oxide may include the sputtering method, the ion plating method (refer to Japanese Laid-Open Patent Application No. 2006-117462), and the thermal decomposition of a precursor (refer to Japanese Laid-Open Patent Application No. 2007-022851).
Generally, when producing quantum wires, dots, etc. having a one-dimensional or two-dimensional periodical minute structure, any of an electron beam exposure system, an ion beam exposure system, and a stepper exposure system is used. However, these exposure systems require the vacuum source and they are expensive, and the production cost becomes high. For this reason, it is desired that simple patterning is performed with low cost and a periodical minute structure is produced.
If a periodical structure in which minute structures are arranged regularly is irradiated by light, unique phenomena, such as the photonic band effect, will arise. Applications of the periodical structure to an optical waveguide, an optical filter, an optical switch, a low threshold laser, etc. which utilize light resonance or photon inclusion, are expected. Moreover, it is known that a periodical structure in which minute structures are arranged regularly at intervals below the subwavelength serves to prevent the Fresnel reflection and exhibits non-reflection characteristics by the structure called moth-eye structure (refer to OPTICAL REVIEW, Vol. 10, No. 2, 2003, pages 63-73).
On the other hand, in the field of biotechnology, it is strongly demanded to use a minute structure as a DNA chip in which molecules and atoms are selectively combined. The DNA chip enables the existence of a gene which is the cause of sickness to be easily investigated, and is used for study on the gene and diagnosis of the sickness.
The DNA chip usually is composed of a thin substrate of silicon or glass, and a DNA (deoxyribonucleic acid) which constitutes the gene which is the cause of sickness is stuck on the DNA chip. If the blood taken from the patient and processed is dropped on the DNA chip and the gene which is the cause of sickness exists in the blood, the DNA in the blood adheres to the DNA of the DNA chip. It is easily judged whether the patient is sick or not. If the behavior of gene is investigated, early detection of sickness and anticipation of the side effects of drugs will be attained. At the medical spots, the demands for genetic screening grow quickly.
Moreover, as for metallic minute structures which are regularly arranged in a two-dimensional formation, the application deployment utilizing interaction of light and molecules by surface plasmon excitation is expected.
The existing method of producing a minute structure and a periodical structure in which minute structures are arranged regularly, uses optical lithography in a semiconductor microfabrication. Since the existing method requires an expensive electron-beam lithography system, it has a problem of high cost. Moreover, the size of the minute structure produced depends on the performance of the producing equipment.
On the other hand, a mask is produced and the existing method has an advantage that it is suitable for mass production. However, it is unsuitable for a simple circuit design at a time of specification change and a simple experiment at an experimental stage.
There is also known a method of producing a three-dimensional minute structure or a three-dimensional photonic crystal using the 2-photon absorption by a laser beam (refer to Japanese Laid-Open Patent Applications No. 2003-001599 and No. 2005-122002). However, the production needs much time and the material used is limited to the resin that can be produced by a photopolymerization reaction. For this reason, a simple, inexpensive method of producing a minute structure which can improve resource saving further is demanded.
The existing method of forming a reversal structure of a minute structure or a periodical structure in which minute structures are arranged regularly uses emboss processing including an injection molding. In recent years, the nano imprint technology which accurately transfers a reversal structure of a nano-scale structure has been developed. The nano imprint technology using photopolymerization or thermal polymerization can produce a reversal structure of a master mold with sufficiently high accuracy, and it is suitable for mass production. When a reversal structure is used as a photonic crystal, a different effect from a master mold arises.
In recent years, attention is given to heat lithography as a low-cost processing method which is more cost-effective than the microfabrication using optical lithography. Heat lithography is a microfabrication technology utilizing the principle that when an endothermic layer is heated (which layer functions as a light absorption layer when irradiated by a laser beam), the characteristics (light transmittance, refractive index, conductivity, chemistry corrosion resistance, etc.) of the heated part are changed. The temperature distribution of the area irradiated by light turns into Gaussian distribution, the area of the high-temperature region in the center of the distribution is about 1/10 of the area of a light spot, and the characteristics of that area only are changed. Thus, fabrication of a minute pattern is possible.
Japanese Laid-Open Patent Application No 2005-158191 discloses a method of manufacturing an optical recording medium which includes at least a step of laminating a first dielectric layer, a light absorption layer, and a second dielectric layer one by one on a supporting substrate, a step of emitting a laser beam to record information, and a step of removing a non-recorded area of the second dielectric layer by wet etching, to form a convex part of the second dielectric layer.
In the method disclosed in Japanese Laid-Open Patent Application No. 2005-158191, the convex part of the second dielectric layer formed has a cross section which is rectangular or in an inverse tapered shape, and since the etching resistance is increased only in the vicinity of the maximum value in the heat distribution (Gaussian distribution), the size of the convex part is less than the diffraction limit of light. And the etching resistance of the 2nd dielectric layer on the light absorption layer where the laser light is absorbed improves and the convex part is formed. However, since there are many uses which require removal of the light absorption layer, forming the convex part, without forming the light absorption layer is demanded. There is also a problem that, when forming a concave part in the second dielectric layer, the end of the concave part is roughed.
An optical element having a subwavelength structure or a fine structure of photonic crystal is demanded in recent years. Application of such fine structure is not limited to optical elements. For example, an organic electro-luminescence (OEL) display or organic light-emitting diode display (OLED) is a new generation light-emitting display using an organic compound. When compared with the conventional display, the light emitted by the OLED is bright and clear, the angle of field is large, the display is of thin type, and the operational temperature range is extensive. The OLED is observed as a display with the outstanding features. Moreover, it is known that the luminous efficiency of OLED is improved by combining it with a two-dimensional photonic crystal structure. For example, refer to “M. Fujita, T. Ueno, T. Asano, S. Noda, H. Ohata, T. Tsuji, H. Nakada and N. Shimoji, Electronics Letters, Vol. 39, p. 1750 (2003)”, “Y. Lee, S. Kim, J. Huh, G. Kim and Y. Lee, Applied Physics Letters, Vol. 82, p. 3779 (2003)”, “M. Kitamura, S. Iwamoto and Y. Arakawa, Japanese Journal of Applied Physics, Vol. 44, p. 2844 (2005)”, “K. Ishihara, M. Fujita, I. Matsubara, T. Asano and S. Noda, Japanese Journal of Applied Physics, Vol. 45, No. 7, p. L210 (2006)”, and “M. Fujita, K. Ishihara, T. Ueno, T. Asano, S. Noda, H. Ohata, T. Tsuji, H. Nakada and N. Shimoji, Japanese Journal of Applied Physics, Vol. 44, p. 3669 (2005)”.
Also in photoelectric conversion devices, such as solar cells, the necessity for fine structure is known. The solar cells are grouped into dry-type solar cells formed of single crystal silicon, polycrystalline silicon, amorphous silicone, etc. and wet-type solar cells, such as Graetzel cell or dye-sensitized solar cell. In the dye sensitized type solar cell, titanium oxide is used as a semiconductor electrode. However, theoretically, solar cells using other oxide semiconductors may be attained and various researches for the purposes are in progress.